1. Field of the Invention
The invention relates generally to an electric parking brake system and a method for controlling the electric parking brake system, and, more specifically, to a control over a force with which a pushing mechanism pushes a friction member against a friction face in the electric parking brake system.
2. Description of the Related Art
Japanese Patent Application Publication No. JP-2004-142517 (JP-A-2004-142517) describes a technology relating to an electric parking brake system. According to the technology, a target value of a friction member pushing force is set at predetermined time intervals and an electric motor is controlled such that the friction member pushing force approaches the target value. In the electric parking brake system described in JP-A-2004-142517, the target value tracking control is executed. When the target value is increased or decreased, the friction member pushing force is increased or decreased in accordance with an increase or a decrease in the target value. For example, when the friction member pushing force becomes smaller than the increase control starting threshold value, which is set based on the target value, the increase control for increasing the friction member pushing force is started. When the friction member pushing force becomes larger than the increase control ending threshold value, the increase control ends. When the friction member pushing force becomes larger than the decrease control starting threshold value, which is set based on the target value, the decrease control for decreasing the friction member pushing force is started. When the friction member pushing force becomes smaller than the decrease control ending threshold value, the decrease control ends. This unfavorably increases the number of times the electric motor is actuated.
Japanese Patent Application Publication No. JP-2001-63537 (JP-A-2001-63537) describes that a target value of a friction member pushing force, which is used when a vehicle is at a standstill, is set to a larger value from among a required pushing force, which is set based on a brake pedal depressing force, and a required pushing force, which is required to maintain the vehicle at a standstill.